I used to be told that most vets can't see an internal US after 7 months of pregnancy, but that was many years ago, when i was doing US regularly and they were only done rectally (no vet that I've had used the probe vaginally). The mares were always palpated first, rectally, before US. At first, even if the mare was found to be pregnant, we did US - as our vet wanted the practice on small mares and finding the pregnancies with her US machine. Later, I was asked if I really wanted an US if the mare was found pregnant. The smallest mare she palpated in those years was a 37" tall Shetland and the largest was a 16.1 hh TB. All were bred to our 45" Shetland stallion and US were done as early as 14 days after hand breeding/live cover.
Now, US can be done externally on a shaved flank/belly, but again it's now been years since our last one, the vet admitted that she wasn't that good at it AND it was still not done any later that about 7/8 months of possibly known dates - our vet considered it a waste of her time/our $$ since there was "so much" in the belly at that time that it might be hard to see/tell... The last time I had an external US done was in 2012. My girlfriend had a palpation done on a 40" Shetland mare in July of 2014 (found to be pregnant) and had been pasture bred April/May. The vet felt it wasn't necessary to go on to an US.
Sorry, not much help.